This document discusses modals and their usage to express degrees of certainty and necessity when discussing Native American rock art in the Southwestern United States. It provides examples of must and have to to express necessity, don't have to to express lack of necessity, and should and ought to to express advisability. It also discusses using may, might, could and other modals to express varying degrees of certainty about the past purposes and meanings of the rock art, which remain largely unknown. Students are provided practice questions to apply these modals in discussing theories about the rock art.